NO20151322A1

NO20151322A1 - Method and reactor to alternate between stationary bed and moving bed for treatment of water, without changing the water level in the reactor

Info

Publication number: NO20151322A1
Application number: NO20151322A
Authority: NO
Inventors: Bjørn Rusten; Jon G Siljudalen; Tomas Vigeland; Gang Xin
Original assignee: Biowater Tech As
Priority date: 2015-10-06
Filing date: 2015-10-06
Publication date: 2017-04-07
Also published as: NO342658B1; WO2017061872A1

Description

METHOD AND REACTOR TO ALTERNATE BETWEEN STATIONARY BED AND MOVING BED FOR TREATMENT OF WATER, WITHOUT CHANGING THE WATER LEVEL IN THE REACTOR

The present invention relates to a method for biological purification of water in a reactor with one or more inlet and outlet zones where water and substrate come into contact with carrier elements for biofilm. The invention also relates to a reactor for carrying out the method and for separation of biofilm sludge.

The reactor can be arranged for aerobic, anaerobic and anoxic purification of municipal and industrial wastewater, process water, water from aquaculture installations and drinking water. The process is based on the principle that biomass is established on a carrier element for the formation of a biofilm. The carrier elements are held in place in the reactor with the help of an outlet arrangement. The degree of filling of the carrier elements in the reactor is so large during normal operation that they can not move freely - hindered movement. All known types of carrier elements, with a specific weight relatively close to the specific weight of water, can be used. Compared to a number of other biofilm processes on the market, the invention will result in a better transfer of oxygen from the air blown into the water and a better transport of water and substrate to the biofilm, something that will result in a more compact and less energy demanding process.

During normal operation, the present invention will trap incoming suspended solids (SS) and produced biomass, collectively referred to as sludge. This sludge will periodically be removed in a unique forward flow washing procedure, where there is no need to change the water level in the reactor and standard influent water is used for washing.

A number of methods for mechanical, chemical and biological treatment of water are known. Biological treatment entails that a culture of microorganisms carries out the desired transformation of the materials in the water. Biological treatment is, to a large extent, combined with mechanical and chemical treatment processes.

Biological treatment is extensively used for treatment of polluted water. Traditionally, biological treatment has been completely dominating for removal of organic matter and, for the last years, biological treatment has also become dominating for removal of nitrogen (nitrification, denitrification, anammox) and relatively common for removal of phosphorus (bio-P removal).

It is common to distinguish between aerobic, anoxic and anaerobic biological processes. In aerobic processes, the microorganisms need molecular oxygen as an electron acceptor. Anoxic processes depend on the absence of molecular oxygen and the microorganisms will use nitrate or sulfate as the electron acceptor. For biological removal of nitrogen an aerobic process, which oxidizes ammonium to nitrate, is combined with an anoxic process that reduces nitrate to molecular nitrogen gas. Anaerobic processes take place in the absence of oxygen and arecharacterized in thatorganic matter in the water is both electron donor and electron acceptor. Anaerobic processes are most relevant for highly concentrated industrial discharge of organic matter, and in a complete decomposition, the end product will be a mixture of methane and carbon dioxide (biogas).

The microorganisms needed for biological treatment can either be suspended in the water phase in a bioreactor, or be attached to surfaces in the bioreactor. A process with suspended microorganisms is called an activated sludge process. The microorganisms in an activated sludge process must be able to create floes that can be separated from the water in a downstream reactor and then returned to the bioreactor. Alternatively, the suspended microorganisms can be held in place in the bioreactor by draining the treated water through membranes with pore sizes so small that the micro-organisms are retained in the bioreactor. This is known as a membrane bioreactor (MBR) process.

A process where the microorganism are attached to a surface is called a biofilm process. Examples of biofilm processes used for water treatment are trickling filters, rotating biological contactors (RBC), submerged biological filters, moving bed processes and fluidized bed processes. Submerged biological filters include both filters with a relatively open biofilm medium of plastic, and filters with small diameter biofilm media (sand, Leca-balls, small polystyrene balls). Submerged biological filters with small diameter media will relatively quickly be clogged with biological sludge and must be regularly tåken out of service for removal of the sludge by backwashing. Submerged biological filters with open biofilm media that are stationary or fixed in place can be operated for a relatively long time with continuous supply of water, but experience has shown that even filters with very large size biofilm media and an open structure will clog up after some time. Because the microorganisms in biofilm processes are fixed on the surface of a carrier material in a bioreactor, the bioreactor itself is independent of downstream sludge separation.

Combinations of processes with suspended microorganisms and processes with fixed microorganisms in the same reactor are known as IFAS (integrated fixed film and activated sludge) processes. IFAS processes have been comprised of activated sludge in combination with either rotating biological contactors, submerged biological filters with an open biofilm carrier medium or moving bed processes.

On a global basis, there are clearly more biological treatment plants with suspended microorganisms, but biofilm processes are becoming more and more popular. Some of the reasons for this are that activated sludge processes have a number of disadvantages. It is often difficult to control the sludge separation. This can lead to large losses of sludge and in the worst case that the biological process collapses, with the associated consequences for the recipient. Another disadvantage is that conventional activated sludge processes need very large volumes for both the bioreactors and for sludge separation in sedimentation basins. However, the advantage of conventional activated sludge processes is that the water is treated in open reactors with no danger of the reactors becoming clogged.

The membrane bioreactor (MBR) process is a relatively new technology, where membranes with very small pore openings are used to separate the activated sludge from the water. With this technology, a significantly higher concentration of microorganisms can be maintained in the bioreactor, which means that the reactor volume will be significantly smaller than in a conventional activated sludge process. Furthermore, the treated water will be free of suspended solids. The disadvantages with this process are that it is still very costly, it requires a high level of pre-treatment in order to remove materials that can lead to clogging of the membranes, the membranes must be cleaned regularly to maintain the hydraulic capacity, and the energy consumption is relatively high.

The traditional trickling filter was the biofilm process that was first tåken into use for treatment of wastewater. Originally, trickling filters were filled with stone, but modern trickling filters are filled with plastic materials with a larger surface area for biofilm growth. Modern trickling filters are relatively tall. The water is pumped to the top of the trickling filter and is distributed evenly over the entire surface. The supply of oxygen takes place by natural ventilation. It is difficult to adjust the amount of water, load of matter and the natural supply of oxygen in a trickling filter so that everything functions optimally. It is relatively common that the biofilm in the upper parts of a trickling filter does not get enough oxygen. Therefore, trickling filters have normally lower conversion rates and require larger reactor volumes than other biofilm processes. To avoid becoming clogged up the biofilm medium must be relatively open and the specific biofilm surface area (m<2>biofilm per m<3>reactor volume) becomes relatively small. This also contributes to an increased reactor volume. Even with an open biofilm medium, clogging and channel formation in trickling filters are well known problems which can be kept under control by ensuring that each part of the trickling filter is frequently subjected to a hydraulic load which is sufficiently high to rinse particulate matter and loosened biofilm out of the trickling filter. In many cases this means that water must be recirculated over the trickling filter. With a height of many meters, energy costs for pumping can be considerable.

The rotating biological contactor (RBC) is a biofilm process that became very popular in the 1970's. The principle is that circular discs with corrugated surfaces are attached to a horizontal shaft that rotates slowly in a tank. The discs are partly submerged in water, and on these discs a biofilm is established that will alternately take up pollutants from the water phase and oxygen from the air when the discs rotate. A big disadvantage with the RBCs is that they are based on prefabricated shafts and discs that make the system not very flexible. All tanks must be sized according to the dimensions of the RBC. It has also been found that there are considerable mechanical problems with the RBCs, often caused by the inability to control the thickness of the biofilm so that the weight becomes too large and the shaft may break or the corrugated discs break. Therefore, very few new RBC plants have been built over the last 25 years.

Submerged biological filters with a relatively open biofilm medium use, in principle, the same type of plastic materials as modern trickling filters. The plastic material is stationary, submerged in the reactor, and oxygen is supplied via diffuser aerators at the bottom of the reactor. A problem with submerged biological filters of this type has been clogging from growth of biomass and formation of channels. Water and air take the path of least resistance and zones are formed in aerated reactors where biomass accumulates and creates anaerobic conditions. Another disadvantage is that there is no access to the aerators below the stationary biofilm medium. For maintenance or replacing of the aerators one must first remove the biofilm medium from the reactor.

Submerged biological filters with a small diameter carrier medium (sand, Leca balls, polystyrene beads) have a very large biofilm surface area. The carrier medium is stationary during normal operation, but this type of filter will clog up with biological sludge and must regularly be tåken out of operation for backwashing and removal of sludge. The process is sensitive to particles in the wastewater, and for wastewaters with a lot of suspended solids the operation cycles between each backwashing become very short. Due to equipment for backwashing and positioning of aerators at the bottom of the reactors, this type of reactors are complicated to construct. A common designation for this type of biofilm reactor is BAF (biological aerated filter) and the best known brand names are Biostyr, Biocarbone and Biofor.

In moving bed reactors, the biofilm grows on a carrier material that floats freely around in the reactor. The carrier material has either been foam rubber pieces or small elements of plastic. Processes that use foam rubber pieces are known by the names Captor or Linpor. The disadvantage with foam rubber pieces is that the effective biofilm surface area is too small because the growth on the outside of the foam rubber pieces dogs up the pores and prevents substrate and oxygen from reaching the inner parts of the foam rubber pieces. Furthermore, sieves have to be installed to prevent the foam rubber pieces from leaving the reactors, and a system which constantly pumps the foam rubber pieces away from the sieves to prevent these from blocking up is also needed. Therefore, very few plants have been built with foam rubber pieces as the biofilm carrier material.

However, in recent years a large number of treatment plants have been built with moving bed processes where the carrier material is small pieces of plastic. The pieces of plastic are normally distributed evenly in the entire water volume and in practical operation the degree of filling is up to a maximum of 67 % based on the bulk volume of biofilm carriers to the total wet volume of the reactor. Sieves keep the plastic pieces in place in the reactors. The reactors are operated continuously, without the need for backwashing. The patent NO 172687 B3 describes that the reactors can be operated with a degree of filling from 30 to 70 %, and that the biofilm carriers have to move freely. The biofilm carriers shall have a specific weight of 0.90 -1.20. The patent also states that there are mixing devices to ensure a complete mixing of the reactor content. It is important that there must be a steady stream of produced sludge to the downstream separation process, so that the particle load becomes much smaller than for separation of activated sludge. It is also pointed out that this is a continuous process, in contrast to biofilter processes with routine backwashing. The process is very f lexible with regard to the shape of the bioreactor. The specific biofilm surface area is higher than for trickling filters and RBCs, but considerably smaller than for BAF processes. However, based on the total volume moving bed processes with carrier material made of small plastic pieces have been found to be as efficient as BAF when taking in to account the extra volume needed for expansion of the filter bed and for backwash water reservoir for the BAF process. Examples of suppliers of moving bed biofilm reactor (MBBR) processes with small plastic pieces as biofilm carrier material are AnoxKaldnes, Infilco Degremont and Hydroxyl Systems.

In a fluidized bed process, the biofilm grows on small grains of sand. The operating principle is based on water being pumped into the bottom of the reactor at such a high flow rate that the sand is fluidized. A very high biofilm surface area is achieved in such systems, and for aerobic processes, it is a challenge to supply sufficient amounts of oxygen. Normally water is recirculated several times in order to get a sufficiently high flow rate to fluidize the sand and oxygen is supplied by saturating the recirculation stream of water with air or pure oxygen. The pumping costs can be high. In full-scale plants, it is difficult to distribute the water in such a way that the entire bed of sand is evenly fluidized. It can also be a problem that the biofilm changes the specific weight of the sand grains so that sand grains with a lot of biofilm fluidizes at a considerably lower flow rate of water than sand grains with less biofilm. Thus, it gets difficult to operate the plant so that one does not loose sand and biomass.

WO 2010/140898 describes a biofilm process where the growth surface for microorganisms consists of carrier elements that are packed so closely that they can not move freely in normal operation, but they have a very limited or a hindered movement. The ideal carrier elements for this process have a large protected surface area and a large pore volume so that the water can easily flow through the carrier elements and ensure good contact between water, substrate and biofilm. During normal operation the carrier bed will accumulate suspended solids from the incoming water as well as from biomass growth. Accumulated solids are periodically removed as sludge during a unique forward flow washing cycle, where incoming water is used for the washing and the water level in the reactor is increased to the point where the wet volume of the reactor is sufficiently large to allow free movement and complete mixing of the biofilm carriers in the reactor. Typical filling degrees of biofilm carriers are about 95 % during normal operation and about 75 % during the forward flow washing cycle. Due to the high filling degree and very limited movement of the carriers during normal operation this process has excellent oxygen transfer and was shown to have more than 1.9 times higher oxygen transfer than the moving bed biofilm reactor in a test at an industrial wastewater treatment plant. It also has very low filtering resistance and negligible head loss over the biofilm carrier bed during normal operation. Several other advantages over other biofilm processes are also pointed out in WO 2010/140898. However, the process has one major drawback that it shares with some of the other biofilm processes, namely the two different water levels during normal operation and during washing. Based on the typical filling degrees mentioned above a reactor with a 5.5 m water depth during normal operation will have to increase the water depth to 7.0 m during the forward flow washing cycle, an increase of 1.5 m. It is a challenge to fit these reactors into the hydraulic profile at existing wastewater treatment plants, especially if using multiple reactors in series.

The present invention is a method and reactor that eliminates the above mentioned drawback by making it possible to change between normal operation (packed bed, stationary bed, limited biofilm carrier movement) and washing mode (fluidized bed, moving bed) without changing (increasing) the water level in the reactor. This will make the reactors fit into the hydraulic profile at most treatment plants and water can normally flow by gravity from the pre-treatment or primary treatment process. With bioreactors in series, forward flow washing can be performed by gravity water flow even when the upstream reactor is in normal mode (stationary bed) operation. All known types of biofilm carrier elements can be used, but carriers with a specific weight relatively near the specific weight of water may be preferred because they can be fluidized with less energy during the washing cycle.

These and other objects and advantages can be achieved by a method for switching between stationary bed and moving bed operation of a biological biofilm process for purification of water, without changing the water level in the reactor, whereby reducing or increasing the reactor volume by positioning one or more fixed volume or inflated members into the reactor in order to reduce the reactor volume to obtain a stationary bed operation and remove the fixed volume member from the reactor or deflate the inflatable member in order to increase the reactor volume to obtain a moving bed operation.

One or more inflatable members are preferably inflated for providing a stationary bed of biofilm carriers in the reactor, and deflated for providing a freely moving bed of biofilm in the reactor, where said fully inflated said bag(s) will occupy between 10 % and 50 % of the wet reactor volume.

Inf lating the inflatable member(s) are preferably done by using compressed air or another gas and opening an adjustable release valve to deflate the inflatable member(s).

Inf lating the inflatable member(s) can also be done by using a liquid, said liquid being pumped through a pipe system and into the inflatable members, or flow by gravity into the inflatable members from a reservoir with a higher liquid level than the reactor, deflate said inflatable members by pumping the liquid out of the inflatable members, or by letting the liquid flow by gravity from the inflatable members through a piping system and an adjustable valve to a reservoir with a lower liquid level than the reactor.

The inflatable members are preferably made of any flexible and/or elastic material that is waterproof, for example, but not limited to, waterproof fabric, rubber or plastic.

The inflatable members can have any kind of shape, for example tapered at the bottom and also on the top, said inflatable members can be attached to a flat wall in a rectangular reactor or to part of the wall in a cylindrical reactor, said inflatable members can also be placed anywhere inside the reactor, for example in the form of a cylinder, a tube, a square box, a rectangular box, or any other shape, and with either a flat or

tapered top or bottom, if the bottom is flat, the entire circumference of the bottom of the member should be firmly attached to the bottom of the reactor to prevent biofilm carriers from getting stuck under the inflatable member, if the bottom is tapered, it is sufficient to secure the center or edge of the inflatable members bottom to the bottom of the reactor, and if the inflatable member have flat tops, they should extend above the water surface, and inflatable members with tapered tops can be either above or below the water surface.

One or more fixed volume members or fixed shape hollow members are preferably submerged in the reactor for providing a stationary bed of biofilm carriers in the reactor, and said fixed members or fixed shape hollow members are either lifted out of the reactor or allowed to float on the surface of the water in the reactor with only a minor submergence, in order to provide a freely moving bed of biofilm carriers in the reactor, where said fully submerged member(s) will occupy between 10 % and 50 % of the wet reactor volume.

The fixed volume members are preferably solid and made of any type of waterproof material, or hollow and made of any material that can provide a rigid structure.

The fixed volume members can have any kind of shape, where the fixed volume members can be tapered at the bottom and also on the top, said fixed volume members can be placed anywhere inside the reactor, for example in the form of a cylinder, a tube, a square box, a rectangular box, or any other shape, and with either a flat, rounded or tapered top or bottom, wherein fixed volume members with flat tops should extend above the water surface when they are at the maximum submergence, wherein the top of fixed volume members with tapered tops can be either above or below the water surface at the maximum submergence.

According to one preferred embodiment, the weight of the solid fixed volume members is used to submerge the fixed volume members, and a crane or other lifting mechanism to float the fixed volume members or lift the fixed volume members out of the reactors.

In an alternative preferred embodiment, the buoyancy of the solid or hollow fixed volume members is used to float the fixed volume members, and a pulley or other mechanism attached at the bottom of the reactor to pull down the fixed volume members when they should be submerged.

In yet another preferred embodiment, a liquid can be used for submerging the hollow fixed volume objects, said liquid can be pumped through a pipe system and into the hollow fixed volume objects, or flow by gravity into the hollow fixed volume objects from a reservoir with a higher liquid level than the reactors, the hollow fixed volume objects are allowed to float by pumping the liquid out of the hollow fixed volume objects or by letting the liquid flow by gravity from the hollow fixed volume objects through a piping system and an adjustable valve to a reservoir with a lower liquid level than the reactor.

Present invention also relates to a reactor for performing the method, comprising one or more influent water pipes and influent water distribution grids, and one or more outlet grids or sieves and outlet pipes for water and sludge, and one or more mixing mechanisms for mixing of the biofilm carriers in moving bed mode when the inflatable members are deflated, and a filling of biofilm carriers that is sufficiently high to make the biofilm carrier bed stationary when the inflatable members are fully inflated and sufficiently low to make the biofilm carrier bed move freely when the inflatable members are fully deflated.

The water flow being either up-flow (from the bottom of the biofilm carrier bed to the top of the biofilm carrier bed), down-flow (from the top of the biofilm carrier bed to the bottom of the biofilm carrier bed), or horizontal-flow (from one side of the biofilm carrier bed to the opposite side of the biofilm carrier bed).

Preferably, the reactor comprises one or more influent water pipes and influent water distribution grids, and one or more outlet grids or sieves and outlet pipes for water and sludge, and one or more mixing mechanisms for mixing of the biofilm carriers in moving bed mode when the solid or hollow fixed volume objects) are floating or lifted out of the reactor, and a filling of biofilm carriers that is sufficiently high to make the biofilm carrier bed stationary when the solid or hollow fixed volume objects are fully submerged and sufficiently low to make the biofilm carrier bed move freely when the solid or hollow fixed volume objects are floating or lifted out of the reactor.

DETAILED DESCRIPTION

The present invention will be described more detailed by means of some preferred embodiments with reference to the drawings, in which;

Figs. 1 to 6 show different aspects of the invention, and

Fig. 7A and 7B show one example of operation of an aerobic reactor in up-flow mode.

The concept of this invention is shown in principle in Figures 1, 2, 3, 4, 5 and 6. A reactor (1) is filled with biofilm carriers (4) and one or more expandable bags (2, 3), shown inflated (2) in Figures 1 A to 5 A and deflated (3) in Figures 1 B to 5 B. The reactor (1) has one or more inlets for water and one or more outlets for water, but these are not shown in Figures 1 to 6. The expandable bags (2, 3) can be made of any flexible and/or elastic material that is waterproof, for example waterproof fabric, rubber or plastic. When fully inflated (see Figures 1 A to 5 A) the expandable bag(s) (2) should prevent free movement of the biofilm carriers (4) and make the reactor (1) into a stationary or packed bed biofilm reactor (1). When the expandable bag(s) (3) are fully deflated the biofilm carriers (4) in the reactor (1) should be able to move freely (see Figures 1 B to 5 B), making the reactor (1) a moving bed biofilm reactor, where the movement is caused by aeration in aerobic reactors (1) or a separate mixing device in anoxic or anaerobic reactors (1).

The present invention also includes an alternative system for changing the liquid volume of the reactor by håving a solid or fixed shape object (15) alternately submerged in the reactor (1) or fully or partly lifted out of the reactor (1), as shown in principle in Figures 6 A and 6 B. The object (15) can be solid and heavier than water so that the object (15) will sink and displace water and biofilm carriers (4) by its own weight (see Figure 6 A), and therefore the object

(15) needs to be lifted out of the reactor (1) by a crane or other lifting mechanism in order to change the reactor (1) into a moving bed reactor (1). The solid object (15) can also be lighter than water, in which case a pulley system or an equivalent mechanism anchored at the bottom will be needed to pull the object (15) down in the water in order to displace water and biofilm carriers (4) and make the reactor (1) into a stationary or packed bed biofilm reactor (1). Releasing the tension that keeps the solid object (15) fully submerged in the water, the solid object (15) will float and be only partly submerged. A lifting mechanism will be needed if the solid object (15) is to be completely lifted out of the reactor (1) in order to change the reactor (1) to a moving bed reactor (1), otherwise the buoyancy of the solid object (15) may be sufficient to increase the water volume to the level where the reactor (1) turns into a moving bed reactor (1).

The fixed shape object (15) can also be hollow with a rigid shell. It can be submerged by pulling it down in the water with a pulley or an equivalent mechanism attached to the bottom of the reactor (1). To increase the water volume and change the reactor (1) from a stationary bed to a moving bed reactor (1) the mechanism that pulls the hollow object (15) down in the water is released so that the hollow object (15) can float. The hollow object (15) can also be submerged by filling it with water or another liquid, and when the water or liquid is siphoned or pumped out of the hollow object (1) it will float with only a minor submergence.

The rigid objects (15), whether they are solid or hollow, may have any shape and can be positioned anywhere in the biological reactor (1). However, the bottom of the objects (15) should preferably be tapered, so that they do not crush biofilm carriers when the objects (15) are lowered from the top of the reactor (1) to the bottom of the reactor (1). The top of the rigid objects (15) should preferably be tapered if the top of the rigid objects (15) are below the water surface in packed bed or stationary bed operation.

How much of the wet volume of a reactor (1) that needs to be occupied by the fully inflated bag(s) (2) or fully submerged rigid object(s) (15) in order to be able to switch between stationary bed and moving bed will depend on the size and shape of the biofilm carriers (4), but it will range from 10 to 50 % of the wet reactor (1) volume, and typically be between 15 and 25 % of the wet reactor (1) volume.

The expandable bags (2, 3) can have any kind of shape, but should preferably have fairly vertical walls. However, they can be tapered at the bottom (Figures 2, 4 and 5) and also on the top (Figure 5). The expandable bags (2, 3) can be attached to a flat wall in a rectangular reactor (1) (Figures 1 and 2) or to part of the wall in a cylindrical reactor (1). The expandable bags (2, 3) can also be placed anywhere inside the reactor (1) (Figures 3, 4 and 5), for example in the form of a cylinder, a tube, a square box, a rectangular box, or any other shape, and with either a flat or tapered top or bottom. If the bottom is flat, the entire circumference of the bottom of the bag (2, 3) should be firmly attached to the bottom of the reactor (1) (Figures 1 and 3) to prevent biofilm carriers (4) from getting stuck under the expandable bag (2, 3). With a tapered bottom it is sufficient to secure the center or edge of the expandable bag (2, 3) bottom to the bottom of the reactor (1) (Figures 2, 4 and 5). Expandable bags (2, 3) with flat tops (Figures 1,2,3 and 4) should extend above the water surface. Expandable bags (2, 3) with tapered tops can be either above or below (Figure 5) the water surface.

Inflation or expansion of the bags (2, 3) can be with either gas or liquid. Air will be the most common gas, and water will be the most common liquid. Gas will create a lot of buoyance and put a lot of stress on the expandable bags (2) and the fasteners used to affix them to the reactor (1), and will be feasible only for fairly shallow reactors (1). When using gas the bags (2) will be inflated by blower or compressor, and deflated through an adjustable release valve with the water in the reactor (1) creating the force to push the gas out of the bags (2,3).

When using liquid for inflating the bags (2) the liquid can either flow by gravity or be pumped into the bags. When deflating the bags (2, 3) the liquid can either be pumped out or siphoned out of the bags (2, 3). Wastewater can be used for inflating the bags (2), but the disadvantage will be that the bags (2) will gradual ly be filled up with impurities and biological activity inside the bags (2) will create anaerobic conditions with odor issues when the bags (3) are emptied.

The preferred method for inflating the bags (2) will be to use dean water, with periodic addition of chlorine or another disinfectant to prevent biological activity in the bags (2, 3). For a real wastewater treatment plant there will normally be multiple biological reactors (1). Depending on treatment objectives there will typically be one to five reactors (1) in series, and most treatment plants will have a minimum of two parallel trains. Most of the time the reactors (1) will be operated with fully inflated bags (2) and a stationary bed of biofilm carriers (4). The time that a reactor (1) is operated with a moving bed of biofilm carriers (4) and deflated bags (3) is fairly short, and at a full-scale treatment plant operation can be staggered so that only one reactor (1) at a time is in the moving bed mode of operation. This means that the volume of the dean water reservoir used for inflating bags (2) can be sized for the volume of the inflated bags in only the largest single biological reactor (1) for a treatment plant with multiple biological reactors (1). Thus, this dean water reservoir will be very small compared to the total biological reactor (1) volume, and the same water is used again and again for inflating bags (2, 3). For a treatment plant with a total of six biological reactors (1), this water reservoir will typically be about 3 % of the total wet bioreactor (1) volume. There must be water pipe connections between all the bags (2, 3) and this reservoir, and pumps that can fill the bags (2, 3) with water and/or empty the bags (2, 3) of water.

The water to be treated in the biological reactors (1) can go through the biofilm carrier (4) bed in an up-flow mode, a down-flow mode or a horizontal-flow mode. The influent water will be introduced into the reactor (1) trough one or more influent pipes and/or influent grids, to be positioned near the top of the reactor (for down-flow mode operation), near the bottom of the reactor (for up-flow mode operation), or on the side of the reactor (for horizontal-flow operation). One or more effluent sieves or grids will be needed to collect the treated water or the excess sludge, and to retain the biofilm carriers (4) in the reactor (1). In up-flow mode horizontal effluent pipe sieves or effluent grids will be positioned near the water surface of the reactor (1). In down-flow mode horizontal effluent pipe sieves or effluent grids will be positioned near the bottom of the reactor (1). In horizontal-flow mode vertical effluent pipe sieves, vertical effluent wall sieves or vertical effluent grids will be positioned near or on the wall opposite the wall where the influent water entered the reactor (1).

Figures 7 A and 7 B show one example of operation of an aerobic reactor (1) in up-flow mode. Similar examples can be shown for reactors (1) with other water flow patterns, as well as for anoxic and anaerobic reactors (1).

In Figure 7 A water has been pumped from the reservoir (11) with a pump (14) through a pipe system (12) so that the expandable bag (2) is fully inflated and the bed of biofilm carriers (4) is stationary. Valve (13) is closed to make sure that the bag (2) stays fully inflated and the water in the bag can not drain back into the reservoir (11). For aerobic operation of the reactor (1) air or pure oxygen is supplied through the air pipe (7) to the aeration grid (8). With tapered bottom on the fully inflated bag (2), the aeration grid (8) can cover the entire bottom of the reactor (1). Water to be treated is supplied through the influent water pipe (5) and the influent water distribution grid (6). Treated effluent is collected in the effluent pipe sieve (9) and discharged through the effluent pipe (10).

When the stationary bed of biofilm carriers (4) has accumulated so much particles and biomass that it is time to remove excess solids in the form of sludge, the reactor (1) must be transformed to a moving bed biofilm reactor (1). This is done by opening the valve (13) on the piping system (12) so that the water in the inflated bag (2) can drain back to the reservoir (11). The water flow rate for draining the bag (2, 3) must be regulated with the valve (13) and should be approximately equal to the influent water flow rate, so that the water level in the reactor (1) does not drop. When the bag (3) is fully deflated (see Figure 7 B), the biofilm carriers (4) will move freely, due to the turbulence created by the air supplied through the aeration grid (8). This turbulence will ensure that the excess solids are suspended. Influent water, entering through the influent water pipe (5) and the influent water distribution grid (6), will push these suspended solids to the effluent pipe sieve (9) and these suspended solids will then exit the reactor (1) as excess sludge through the effluent pipe (10). When enough water has passed through the reactor (1) to remove a sufficient amount of sludge, the reactor (1) is transformed back to a reactor (1) with a stationary bed of biofilm carriers (4). This is done by pumping (14) water from the reservoir (11) and through the piping system (12) until the bag (2) is fully inflated. When the bag (2) is fully inflated the pump is stopped and the valve (13) is closed.

In aerobic operation the mixing in moving bed mode is normally done by aeration through the aeration grid (8), but additional mixing may be provided by separate mixing devices. Such separate mixing devices include, but are not limited to, circulation pumping with one or more pumps or mixing with propellers or stirrers. In anoxic or anaerobic operation mixing in moving bed operation is provided by separate mixing devices, and such separate mixing devices include, but are not limited to, gas mixing, circulation pumping with one or more pumps or mixing with propellers or stirrers.

Claims

1. Method for switching between stationary bed and moving bed operation of a biological biofilm process for purification of water, without changing the water level in the reactor (1),characterized byreducing or increasing the reactor (1) volume by positioning one or more fixed volume or inflated members into the reactor in order to reduce the reactor (1) volume to obtain a stationary bed operation and remove the fixed volume member from the reactor (1) or deflate the inflatable member in order to increase the reactor (1) volume to obtain a moving bed operation.

2. Method according to claim 1,characterised in thatone or more inflatable members (2, 3) are inflated for providing a stationary bed of biofilm carriers (4) in the reactor (1), and deflated for providing a freely moving bed of biofilm carriers (4) in the reactor (1), where said fully inflated said bag(s) (2) will occupy between 10 % and 50 % of the wet reactor (1) volume.

3. Method according to claim 2, characterized byinflating the inflatable member(s) (2), using compressed air or another gas and opening an adjustable release valve to deflate the inflatable member(s) (2).

4. Method according to claim 2, characterized byinflating the inflatable member(s) (2) by using a liquid, said liquid being pumped (14) through a pipe system (12) and into the inflatable members (2, 3), or flow by gravity into the inflatable members (2, 3) from a reservoir with a higher liquid level than the reactor (1), deflate said inflatable members (2, 3) by pumping the liquid out of the inflatable members (2, 3), or by letting the liquid flow by gravity from the inflatable members (2, 3) through a piping system (12) and an adjustable valve (13) to a reservoir (11) with a lower liquid level than the reactor (1).

5. Method according any of the claims 1-4,characterized in thatthe inflatable members (2) are made of any flexible and/or elastic material that is waterproof, for example, but not limited to, waterproof fabric, rubber or plastic.

6. Method according any of the claims 1-5,characterized in thatthe inflatable members can have any kind of shape, for example tapered at the bottom and also on the top, said inflatable members (2, 3) can be attached to a flat wall in a rectangular reactor (1) or to part of the wall in a cylindrical reactor (1), said inflatable members (2, 3) can also be placed anywhere inside the reactor (1), for example in the form of a cylinder, a tube, a square box, a rectangular box, or any other shape, and with either a flat or tapered top or bottom, if the bottom is flat, the entire circumference of the bottom of the member (2, 3) should be firmly attached to the bottom of the reactor (1) to prevent biofilm carriers from getting stuck under the inflatable member (2, 3), if the bottom is tapered, it is sufficient to secure the center or edge of the inflatable members (2, 3) bottom to the bottom of the reactor (1), and if the inflatable member (2, 3) have flat tops, they should extend above the water surface, and inflatable members (2, 3) with tapered tops can be either above or below the water surface.

7. Method according to claim 1,characterized bysubmerging one or more fixed volume members (15) or fixed shape hollow members (15) in the reactor (1) for providing a stationary bed of biofilm carriers (4) in the reactor (1), and said fixed members (15) or fixed shape hollow members (15) are either lifted out of the reactor (1) or allowed to float on the surface of the water in the reactor with only a minor submergence, in order to provide a freely moving bed of biofilm carriers (4) in the reactor (1), where said fully submerged member(s) (15) will occupy between 10 % and 50 % of the wet reactor (1) volume.

8. Method according to claim 7,characterized in thatthe fixed volume members (15) are solid and made of any type of water proof material, or hollow and made of any material that can provide a rigid structure.

9. Method according to any of the claims 1, 7 and 8,characterized inthe fixed volume members (15) have any kind of shape, where the fixed volume members (15) can be tapered at the bottom and also on the top, said fixed volume members (15) can be placed anywhere inside the reactor (1), for example in the form of a cylinder, a tube, a square box, a rectangular box, or any other shape, and with either a flat, rounded or tapered top or bottom, wherein fixed volume members(15) with flat tops should extend above the water surface when they are at the maximum submergence, wherein the top of fixed volume members (15) with tapered tops can be either above or below the water surface at the maximum submergence.

10. Method according to any of the claims 1 and 7-9,characterized byusing the weight of the solid fixed volume members (15) to submerge the fixed volume members (15), and a crane or other lifting mechanism to float the fixed volume members (15) or lift the fixed volume members (15) out of the reactors (1).

11. Method according to any of the claims 1 and 7-10,characterized byusing the buoyancy of the solid or hollow fixed volume members (15) to float the fixed volume members (15), and a pulley or other mechanism attached at the bottom of the reactor (1) to pull down the fixed volume members (15) when they should be submerged.

12. Method according to any of the claims 1 and 7-11,characterized byusing a liquid for submerging the hollow fixed volume objects (15), said liquid can be pumped through a pipe system and into the hollow fixed volume objects (15), or flow by gravity into the hollow fixed volume objects (15) from a reservoir with a higher liquid level than the reactor (1), the hollow fixed volume objects (15) are allowed to float by pumping the liquid out of the hollow fixed volume objects (15) or by letting the liquid flow by gravity from the hollow fixed volume objects (15) through a piping system and an adjustable valve to a reservoir with a lower liquid level than the reactor (1).

13. Reactor (1) for aerobic, anoxic or anaerobic purification of water, with one or more inflatable members (2, 3) according to claims 1, 3 and 4, characterized byone or more influent water pipes (5) and influent water distribution grids (6), and one or more outlet grids or sieves (9) and outlet pipes (10) for water and sludge, and one or more mixing mechanisms for mixing of the biofilm carriers (4) in moving bed mode when the inflatable members (3) are deflated, and a filling of biofilm carriers (4) that is sufficiently high to make the biofilm carrier (4) bed stationary when the inflatable members (2) are fully inflated and sufficiently low to make the biofilm carrier (4) bed move freely when the inflatable members (3) are fully deflated.

14. Reactor (1) according to claim 13,characterized bythe water flow being either up-flow (from the bottom of the biofilm carrier (4) bed to the top of the biofilm carrier (4) bed), down-flow (from the top of the biofilm carrier (4) bed to the bottom of the biofilm carrier (4) bed), or horizontal-flow (from one side of the biofilm carrier (4) bed to the opposite side of the biofilm carrier (4) bed).

15. Reactor (1) according to any of the claims 13 and 14, characterized byone or more influent water pipes (5) and influent water distribution grids (6), and one or more outlet grids or sieves (9) and outlet pipes (10) for water and sludge, and one or more mixing mechanisms for mixing of the biofilm carriers (4) in moving bed mode when the solid or hollow fixed volume objects (15)) are floating or lifted out of the reactor (1), and a filling of biofilm carriers (4) that is sufficiently high to make the biofilm carrier (4) bed stationary when the solid or hollow fixed volume objects (15) are fully submerged and sufficiently low to make the biofilm carrier (4) bed move freely when the solid or hollow fixed volume objects (15) are floating or lifted out of the reactor.

16. Reactor (1) according to claim 14,characterized bythe water flow being either up-flow (from the bottom of the biofilm carrier (4) bed to the top of the biofilm carrier (4) bed), down-flow (from the top of the biofilm carrier (4) bed to the bottom of the biofilm carrier (4) bed), or horizontal-flow (from one side of the biofilm carrier (4) bed to the opposite side of the biofilm carrier (4) bed).